1. Field of the Invention
The invention relates to a device for joining at least two material flows in a laminar coextrusion machine.
2. Description of Related Art
Extrusion machines are used to transform plastic raw material into a sheet that can then be worked.
The plastic materials known at this time have specific characteristics.
Also, there is no single plastic material which combines all the characteristics desired for a particular use.
For example, some materials have good mechanical strength while others are resistant to external factors, such as ultraviolet rays, and still others are particularly suitable for working at high or low temperatures.
In order to obtain an object having the desired characteristics, it is known to produce multilayered products by means of a process known as coextrusion.
There are two types of coextrusion, one called "manifold" coextrusion which consists of using, for each material, an extruder and as many extrusion heads as there are layers to be constituted.
These heads converge on an extrusion die where the layers are joined.
With this type of coextrusion, each material is handled separately along its entire path, which makes it possible to use extremely different materials with widely varied melting points, but operation is very complex and the material very expensive.
The other coextrusion process, which is the one to which the invention relates, is the process known as laminar extrusion.
In this process, the joining of the layers takes place in a channel disposed upstream from a single extrusion head in a tool called an extrusion box.
For this reason, the different layers must move at the same speed and continuously in order to prevent any shear.
It is therefore necessary to obtain a laminar flow in each layer.
In order to obtain and maintain this laminar flow, it is necessary to reduce the causes of disturbances, that is mainly, the losses of pressure that result in changes in the cross-section of the channels for transporting the flows and in their directions.
A laminar coextrusion unit of this type comprises:
at least two extruders and, more precisely, as many extruders as there are layers of different materials to be joined, PA1 downstream from the extruders, a device for joining the different flows issuing from the extruders into a so-called global flow, this device also being called a distribution box, and PA1 downstream from this joining device, an extrusion head supplied by the global flow. PA1 at least two extruders, PA1 a device for joining the flows issuing from the extruders into a global flow, and PA1 an extrusion head supplied by the global flow,
This joining device essentially comprises a main channel into which opens, for each material flow, a so-called supply channel connecting each end to the main channel.
In the first coextrusion systems, the flow of a first viscous material was introduced into this extrusion box in the form of a cylindrical rod which constituted the core of the product, and the other flows conformed to the circular cross-section of the flow up to its outlet, where all of the flows were then transformed into a planar sheet with the aid of a geometric transformation means.
This geometric transformation means was essentially comprised of a so-called geometric transformation channel of rectangular cross-section which was perpendicular to the axis of movement of the rod downstream from the main channel and wherein the cross-section of this geometric transformation channel increased in at least one of the two dimensions of its cross-section, and in the direction of movement of the flow, until the desired cross-section was obtained.
This geometric transformation on output is not easy to obtain given that this is a multi-component product with different viscosities and properties.
Thus, it was difficult to conform to the different tolerances of each layer.
In order to eliminate the problems of the geometric transformation of a multi-layer rod, it is known to preform each layer before they are joined, and for this reason, the above-mentioned geometric transformation means is placed in the path of each extruder flow and upstream from the point at which these flows are joined (U.S. Pat. No. 5,137,675).
Thus, extrusion devices become extremely complicated when it is necessary to equip the extruders with a multiplicity of parts designed to be removable for cleaning.
The devices are even more complicated when each supply channel of the joining device comprises means for adjusting the flow.
In the known solutions, the means for adjusting the flow essentially comprise a restricting bar which is placed across the flow in order to modify the cross-section of flow of the supply channel.
At the present time, these adjusting means are placed at a non-negligible distance from the plane in which the supply channel of the extruder in question connects to the channel for joining the flows.
Because of this situation and the means used, it is not possible to increase the number of layers, and the known solutions are generally limited to one central layer and one layer on each side.
Moreover, while the axis of the extruder of the main rod is within the axis of the extrusion, that is, horizontal, this is not the case with the other extruders whose supply is vertical, thus increasing the lengths of the channels and the number of changes of direction, which considerably increases the losses of pressure.